Keyboard-operated electronic instruments of the digital tone synthesizer type are well-known. In the co-pending application U.S. Ser. No. 603,776, filed Aug. 11, 1975, entitled POLYPHONIC TONE SYNTHESIZER and now U.S. Pat. No. 4,085,644 there is described a keyboard instrument in which a plurality of tone generators are provided, each tone generator generating a musical tone from a master data list. The data list represents the amplitude values of equally spaced points along an analog signal corresponding to one cycle of the musical tone to be generated. The master data list for each tone generator is stored in a shift register, and the amplitude values are shifted out of the register to a digital-to-analog converter at a shift frequency which is directly proportional to the fundamental frequency of the note being generated.
As described in the above-identified co-pending application, the shift frequency is derived from a variable frequency oscillator. The frequency of the oscillator is controlled by depressing a key on the keyboard. An assignor circuit stores the note identification in a memory and assigns a tone generator to that particular key. The note identification operates as an address of a memory storing separately addressable frequency control numbers.
The frequency of the oscillator is set according to the frequency number read out of the memory in response to the particular key on the keyboard which is despressed. Each tone generator in the instrument has its own oscillator. This permits a number of notes to be generated at the same time, each at a different pitch or frequency, as in sounding a chord. The manner in which the multiple oscillators are controlled is described in more detail in the co-pending application U.S. Ser. No. 634,533 filed Nov. 24, 1975, entitled FREQUENCY NUMBER CONTROL CLOCK, now issued as U.S. Pat. No. 4,067,254. The manner in which the keys in the keyboard are assigned to the tone generators is described in more detail in the co-pending application U.S. Ser. No. 619,615 filed Oct. 6, 1975 entitled KEYBOARD SWITCH DETECT AND ASSIGNOR, now issued as U.S. Pat. No. 4,022,098.
One problem in the use of a plurality of variable frequency oscillators is that of maintaining the instrument in properly tuned condition. Each oscillator must accurately reproduce the frequency of each and every note in the diatonic scale or some integral multiple thereof. However, variable frequency oscillators tend to drift in frequency with time. Also, changes in ambient conditions can affect their frequency. The oscillators must be adjusted so that each key on the keyboard will set any assigned oscillator to the same nominal frequency. Otherwise the pitch of a note will vary depending on which tone generator is assigned to the particular key. This requires extremely stable oscillators that can be accurately set to oscillate over a very wide frequency range. These conditions are a bit difficult to achieve at reasonable cost.
For this reason it would be desirable to generate the pulse trains for shifting the shift registers of the respective tone generator from a single master clock pulse source. One method heretofore proposed for synthesizing clock pulse trains at the musical frequencies is to employ what is called a "top octave synthesizer" having a set of integer counters, one counter for each of the 12 notes in the octave scale. These counters produce an integer division of a single master clock. To produce clock trains corresponding to the frequencies in the top octave requires a master clock rate of approximately 2 megahertz. However, in the polyphonic tone synthesizer described in the above-identified co-pending application U.S. Ser. No. 603,776, now U.S. Pat. No. 4,085,644, the shift pulse frequency must be 64 times the frequency of the note being generated. This would require a master clock frequency which is far too high for the present state of the art.
An alternative technique for obtaining a plurality of frequencies by frequency division from a common clock source is to use a non-integer divider. However, non-integer dividers, while producing pulse trains of any desired average frequency, generate pulse trains in which the intervals between pulses are not always identical. The number of pulses occurring over a given period of time is varied by eliminating pulses at selected pulse intervals from the pulse train. However, if a non-integer divider were used to generate the shift pulse trains in tone generators in a polyphonic tone synthesizer of the type described in the above-identified application U.S. Ser. No. 603,776, now U.S. Pat. No. 4,085,644, the unequal spacing of the pulses in the pulse train would introduce a highly objectionable noise into the system. This noise would be in the form of audible higher frequency components which, because of their wide separation from the fundamental frequency and high level of intensity, would produce displeasing tonal effects.